Simplified: How LIGO detects gravitational waves?

The Laser Interferometer Gravitational-Wave Observatory or LIGO is a large-scale physics experiment and observatory constructed and operated to detect gravitational waves. The mission of this project is to directly observe gravitational waves of cosmic origin.

Central to the technology behind LIGO is laser interferometry. In physics, interference transpires when two waves carrying energy meet up and overlap. Instead of having two waves, this overlapping of energy results in a third wave whose shape and size depend on the patterns of the original two waves. Superposition is term used to describe this process involving the combination of two waves.

Interferometry makes use of the principle of superposition to combine waves in a way that will cause the result of their combination to have some meaningful property that is diagnostic of the original state of the waves.

Furthermore, interferometry uses an interferometer—a device that separates a beam of light into two ray beams, usually by means of reflection, and that brings the rays together to produce interference, used to measure wavelength, index of refraction, and astronomical distances. LIGO simply works using the principle of interferometry. The LIGO is actually a large-scale laser interferometer designed specifically to detect gravitational waves.

A brief history of LIGO

The United States National Science Foundation or NSF provided support and funding for the study and construction of LIGO starting in 1979. The NSF specifically provided a funding for the study of a large interferometer led by Massachusetts Institute of Technology or MIT. California Institute of Technology or Caltech also started working on the construction a 40-metre prototype in 1981.

Meanwhile, the MIT study determined the feasibility of constructing and operating an interferometer as long as one kilometre. But the NSF pressured both MIT and Caltech and the two institutions subsequently joined forces.

There were several setbacks in the project due to the combination of technical shortcomings and organisational problems. Scientists had a hard time perfecting the specifications for LIGO while also struggling with internal and external politics. These problems affects the progress of LIGO.

Construction nonetheless began in 1994 in Hanford, Washington and Livingston, Louisiana. The project became fully operational in 2002 when it started searching for evidence of gravitational waves. The search however ended in 2005 after five attempts. Researchers gathered that the sensors needed upgrading to improve sensitivity.

The so-called Enhanced LIGO was completed in 2009 and the hunt for the elusive gravitational waves restarted. By 2010, the search yielded no result.

A new major upgrade began and the resulting product was the Advanced LIGO. This was completed and introduced in 2014. This iteration to the LIGO has four times the sensitivity of the original version.

The year 2015 marked the beginning of another attempt to search for gravitational waves. In September 2015, the Advanced LIGO detected a signal that appeared to come from the collision of two black holes. A thorough analysis of the data revealed that gravitational waves were finally detected. The announcement was made in February 2016.

Detecting gravitational waves using LIGO

The strongest ripples or gravitational waves should come from massive objects changing configurations at high velocity. Examples of these include gravitational collapse leading to the formation of black holes, the condensation of matter before galaxy formation, a neutron star and a black hole pair, and a pair of neutron starts or black holes orbiting one another. But even these strongest waves are still weak to be easily detected by the time they reach Earth.

It is also important to note that measuring the physical changes caused by the distortion in the spacetime fabric is tricky. For a simplified description, observing how far a rubber sheet has stretched by observing the distance between placed markers is useless. These markers also stretch with the rubber sheet and they would create an illusion that nothing has changed.

The best way to measure the distortion or to observe gravitational waves is to use the speed of light. This speed does not stretch. If the space between two points stretches, light would take longer to go from one point to another. If the space stretches back or squeezes in, light would take less time to go back-and-forth from the two points.

This is where the principle of interferometry and LIGO come in the picture. The concept behind LIGO centres on the use of massive laser interferometers located thousands of kilometres apart to exploit the physical properties of light and of space itself to detect and understand the origins of gravitational waves

The entire observation includes an L-shaped tunnel that uses laser and mirrors placed at the two ends of the tunnel. LIGO fires and split laser from the centre or vertex of the L-shaped tunnel for two laser beams to travel into two paths—one for each arm of the L. Each laser beam sent to the two ends of the tunnel would be reflected back to the centre. The laser fundamentally measures the changes in the distance between the ends of the tunnel.

When gravitations waves pass through the entire LIGO, the spacetime fabric stretches in one direction and stretches back or squeeze in another direction. This stretching or strain in the spacetime fabric would create discrepancies in distance between the ends of the tunnel. This is because the strain would change the timing of when the split laser beams travel to back and forth to their destination.

These discrepancies reflect the stretching and squeezing from two directions. Furthermore, These discrepancies would mark the existence of gravitational waves. LIGO essentially records these discrepancies after measuring the fluctuating distance caused by the stretching and squeezing of the spacetime fabric.

A simplified description of how LIGO works to detect gravitational waves involves describing the entire thing as a massive instrument made of two lasers. Based on the concept of interferometry, these lasers measure the changes in the size of plane or a flat two-dimensional surface. Take note that this plane is just a simple representation of spacetime fabric. Gravitational waves stretches and squeezes the plane, causing it to expand or shrink in size. Lasers can accurately measure this expansion or shrinkage in the plane using the speed of light as a reference. An expanded plane would make light to travel longer from one side to another as opposed to a shrunk plane.

The entirety of LIGO continuously measures how long and how short the light travels from each side of the plane. It records the measurements over time to see any discrepancies. The presence of these discrepancies indicate gravitational waves passing through a plane. LIGO simply looks for these discrepancies in the size of the plane or an area to determine a distortion in the spacetime fabric caused by gravitational waves.